For making mechanical components, such as gears, bearing components and shafts to be used in automobiles, JIS steel species like SCR420 is generally employed after having been worked into the shapes of intended components and then subjected to surface hardening treatment including carburizing and quenching with the intention of enhancing abrasion resistance, fatigue strength and the like.
The carburizing-and-quenching treatment is high-temperature, long-duration heat treatment, and tends to cause coarsening of crystal grains. For this reason, various studies and proposals to prevent the coarsening of crystal grains have been made.
A technique of pinning grain boundaries by precipitating particles of AlN or the like in a dispersed state at a manufacturing step before carburization has been widely adopted as a useful technique for preventing crystal grains from becoming coarse.
For example, techniques of this kind have been disclosed in Patent Documents 1 and 2 cited hereafter.
However, the techniques of such a kind which allow pinning of grain boundaries by utilizing precipitate particles are incapable of properly preventing an abnormal grain growth phenomenon that abnormal coarsening of crystal grains occurs locally.
The term abnormal grain growth used herein refers to a phenomenon occurring for a reason that, though a pinning force of precipitate particles was greater than a driving force for crystal grain growth in the initial carburizing stage, the magnitude relation between these forces comes to reverse, or equivalently, the driving force for crystal grain growth becomes greater than the pinning force of precipitate particles, in the course of the carburizing, and such reversal of force relation takes place through a cause that the pinning force is reduced by solid solution being formed from precipitate particles under the carburizing, by precipitates being coarsened through Ostwald growth, and so on.
As to components given cold forging, on the other hand, a distribution of plastic distortions is introduced into the interior thereof at the time of the forging, and a reversal of magnitude takes place between pinning force and driving force of crystal grain growth in locations greater in distortion under carburizing, thereby causing abnormal grain growth in crystal grains.
FIGS. 2 (A) to 2 (C) are showing model-wise the appearance of abnormally grown grains.
FIG. 2 (A) shows a state at the initial stage of carburization, and p represents a precipitate particle (a pinning particle). In the state at the initial stage of carburization, many precipitate particles p lie at grain boundaries, and pin and restrain boundaries between crystal grains q, thereby inhibiting the crystal grains q from growing to a larger size.
However, part of precipitate particles p pinning grain boundaries disappear by forming a solid solution under carburizing, and herein the pinning (restraint) by such precipitate particles p is broken (comes undone), and some adjacent pairs of crystal grains thus made free from the pinning at their boundaries tend to coalesce and grow into one crystal grain.
Crystal grains which have increased in size in such a way can gain power for grain growth, and under relative reduction in the pinning force of precipitate particles p, each crystal grain breaks the crystal grain boundary pinning by precipitate particles and swallows one neighboring crystal grain after another, thereby continuing its grain growth.
More specifically, once the grain boundary pinning by precipitate particles p has been broken, the pinning-broken crystal grain boundaries function as the center of grain growth, and from such crystal grain boundaries the grain growth occurs chain-reactionally to develop into abnormal grain growth and finally form giant-sized crystal grains Q as shown in FIG. 2 (B).
FIG. 2 (C) shows an example of abnormally-grown grains (a photograph of crystal grains after carburization).
Incidentally, the photograph of this example is a shot of the central portion of a steel material having undergone carburizing treatment at 1,100° C., and this material is listed as Comparative Example 1 in Table 1 shown hereafter.
When such abnormal grain growth has occurred, heat treatment distortion develops due to local enhancement of hardenability, and thereby causes problems of making noises and vibrations or reducing fatigue strength.
In such a case has hitherto been taken a measure that greater many precipitate particles are made to precipitate in a dispersed state, thereby further enhancing the power of grain boundary pinning by precipitate particles. However, occurrence of the abnormal grain growth cannot be prevented to a sufficient degree by such a measure.
Recent years in particular have seen widespread use of a technique of raising carburization temperatures for the purpose of reducing carburizing hours, a technique of conducting cold forging for reduction of manufacturing costs of components and techniques adaptable to environmental protection, such as vacuum carburization performed for the purpose of reducing evolution of CO2 in the course of manufacture and enhancing the strength, but the abnormal grain growth has been more likely to occur under those techniques. Thus there have been demands for measures allowing effective inhibition of such abnormal grain growth.
Additionally, as another previous art relating to the present invention, an invention of “a case hardening steel superior in cold workability and crystal grain coarsening characteristics” has been presented in Patent Document 3 cited hereafter, and this document has disclosed the point that, because AlN particles currently in use for pinning grain boundaries formed a solid solution or increased in size at temperatures of 900° C. or higher and thereby were unable to produce much effect on prevention of crystal grain coarsening during the carburizing treatment, the prevention of grain coarsening was attempted by doping a steel material with Nb and Al and causing these elements to combine with C and N, thereby forming fine combined precipitates.
However, the invention disclosed in Patent Document 3 is basically different from the present invention in a point that the doping with an excessive amount of Nb was carried out therein in contrast to the present invention avoiding doping with Nb as an impurity.
As still another previous art relating to the present invention, an invention of “a case hardening steel superior in crystal grain-coarsening resisting properties, fatigue characteristics and machinability, and a manufacturing method thereof” has been presented in Patent Document 4 cited hereafter, and this document has disclosed the point that, without impairing the crystal grain-coarsening resisting properties, improvements in fatigue characteristics and machinability were made by properly adjusting the size distribution of Ti precipitates in the steel.
However, the substance of the disclosure made in Patent Document 4 consists in precipitating 10 or more Ti precipitates per mm2 having a size of 1.0 to 5.0 μm, and all the steel materials 1 to 26 according to the invention disclosed in Patent Document 4 an excessive amount of Ti compared with an amount of N and do not fall within the scope of the expression (1) in the present invention. The invention disclosed in Patent Document 4 is therefore different from the present invention.
As the other previous art relating to the present invention, an invention of “a steel for use in carburized parts which is superior in cold workability, allows prevention of crystal grains from coarsening during the carburization and has excellent impact-resisting properties and impact fatigue-resisting properties” has been presented in Patent Document 5 cited hereafter, and this document has disclosed the point that Ti or both Ti and Nb were incorporated into steel in such amounts as not to impair cold workability and machinability and made to precipitate out in the form of carbides or nitrides thereof, thereby allowing prevention of crystal grain coarsening during the carburization.
More specifically, Patent Document 5 discloses that the Ti content is limited to 0.1% to 0.2%, the N content is limited to 0.01% or less and the Al content is limited to 0.005% to 0.05%, and in Examples 1 to 11 disclosed therein, an excessive amount of Ti is doped compared with an amount of N, in terms of mole ratio, in such a way that TiC is precipitated. The concept of this disclosure is therefore opposite to that of the present invention and outside the scope of the expression (1) in the present invention. Moreover, Patent Document 5 further discloses that the Ti content is limited to 0.025% to 0.05%, the Nb content is limited to 0.03% to 0.2%, the N content is limited to 0.01% or less and the Al content is limited to 0.005% to 0.05%, which situation is different from that of the present invention in the point that an excessive amount of Nb is doped.
Patent Document 1: JP-A-2001-303174
Patent Document 2: JP-A-H8-199303
Patent Document 3: JP-A-H9-78184
Patent Document 4: JP-A-2007-31787
Patent Document 5: JP-A-2006-213951